The structural behavior of Al 3+ in peralkaline melts and glasses in the system Na 2 O-Al 2 O 3 -SiO 2

The structural behavior of Al 3+ in peralkaline glasses and melts along the Na 2 Si 3 O 7 -Na 2 (NaAl) 3 O 7 join has been examined to 1200 °C at ambient pressure with 29 Si MAS NMR and Raman spectroscopy. The distribution of Al 3+ among coexisting Q 4 , Q 3 , and Q 2 structural units in the glasses and melts was determined as a function of bulk Al/(Al + Si) and temperature. The Al 3+ resides principally in Q 4 structural units, which contain more than 70% of the total amount of Al 3+ . The Q 2 units contain the smallest amount of Al 3+ among the Q 4 , Q 3 , and Q 2 structural units. There is no evidence for temperature-dependent distribution of Al 3+ among the coexisting structural units at least to 1100- 1200 °C. The equilibrium constant for the speciation reaction, 2Q 3 ↔ Q 4 + Q 2 , passes through a maximum with increasing bulk melt Al/(Al + Si) at constant temperature. Its temperature-dependence also depends on bulk melt Al/(Al + Si) so that the enthalpy of the speciation reaction, △H, passes through a maximum with increasing bulk Al/(Al + Si). This △H ranges between 13 and 23 kJ/mol for bulk melt Al/(Al + Si) between 0 and 0.33. These composition-dependent maxima in the equilibrium constant (at constant temperature) and △H of the speciation reaction reflect the changes in Al/(Al + Si) of Q 2 , Q 3 , and Q 4 structural units with Al/(Al + Si) of the melt.